From Cities

In the Undercroft of the castle you will find rainbow guards. Kill the guards in order (starting with an infra-red guard and working your way up the spectrum to an ultra-violent guard). Your character box will change colour to reflect how far up the spectrum you've got. If you kill anything out of order you'll have to start again - so make sure to have a wand of your alignment ready so that you can skip past higher-level guards without having to fight them and reset your progress.

As you kill the Ultra-Violent Guard, you hear a voice from behind you...
"You're good at this. If you ever get bored with it, come down to the dungeons,
and my prisoner will teach you something more interesting."
You spin around, but nobody is there.

In the northern section of the Undercroft is a staircase down. Initially it leads to a pointless lower level with just one room, but now it leads to the dungeon, where Master Smith Wayland is a prisoner. For 2 days worth of AP, he will train you to be a Smith. Also, your Astrolabe can now predict farther into the future to determine where the Ancient Castle will be.

Wrinkles:

The final (ultra-violent) guards are invisible, have 1000HP, and hit hard (55 damage a strike). If you don't have a Diamond Bling or some other equippable item that allows you to see invisible monsters, it will take a long time to kill one of these guards, since you won't be able to align it or use your heavy or ranged weapons, and you'll have to keep stopping to heal yourself.

The lower level guards (infra-red to green) drop useful fruit (needed for the Totems quest, for instance). If you're thinking of collecting fruit from them, do it before you become an armourer, because you won't be able to attack them at all afterwards - not even with a cruel blade or wand of anger: they simply won't attack Armourers.